This is the latest incarnation of possibly the most detailed and comprehensive fantasy setting ever: Hârn. It's a place I've been prowling - or at least my characters have but it's so realistic it's easy to forget that you haven't visited there yourself! - for many a year, even though in one memorable game my bard was kicked out of the city of Thay and told never to return! The Introduction explains the history of this setting from its first appearance in 1983 to the present, and lays out how it is organised. This book is itself an introduction, an overview. In some ways the real meat, the detail that makes the setting so glorious, is to be found in Hârndex and in the component articles of the Encyclopaedia Hârnica - these articles are available as looseleaf punched pages (or PDFs) enabling you to collect just the ones you need. (You'll probably end up wanting all of them, Hârn is addictive!)
It starts with an overview of Hârn itself, a large island, a wild and dangerous land, where pockets of civilization are surrounded by large tracts of wilderness. Those pockets of civilisation are mostly feudal kingdoms, but apart from the squabbling that is inevitable when you have lots of lordlings each with an armed retinue, there are religious quarrels, barbarian hordes, unexplored expanses and much more to keep your characters occupied. Oh, and there are elves, dwarves, orcs and even stranger races to be found there as well. Maps abound - both in the book itself and the large one that accompanies it - and serve to make the place become 'real'. It's hard to remember that it isn't lurking somewhere nearby, just waiting for you to visit.
Overview done, there's a section on Culture, wich consists of sub-sections summarising each of the kingdoms and other groupings to be found on Hârn. Each provides a brief history, notes on government, economy and more, along with the coat of arms (heraldry is big on Hârn - and yes, there's a supplement on it if you want to learn more) and a list of the separate articles that particularly contribute to knowledge of that area. These are followed by sub-sections covering tribes and other races.
Next is a section on Government (although this has been touched on briefly already). Here is a good explanation of what a feudal system actually is, and how such a society is structured. It's to be remembered that in a true feudal system, the lord has just as many obligations as the vassal who owes him service - the rights and obligations operate in both directions. Good lords realise this and take their duty of care and obligations seriously... but some do not, and it can be quite hard to rid yourself of such a tyrant! There are details of what rights and responsibilities are in the monarch's purview, and what constitutes the royal court: offices and responsibilities. Then the discussion passes on to shires and eventually down to manors, the smallest division of land, showing how everything (and everybody) interlink.
Not everyone is settled on the land, however, so the next section looks at Cities. There are eight on Hârn, although most are pretty small. Only about 10% of the population lives in a city, and the largest has about twelve thousand citizens. Again the governance and structure is discussed, with the various offices and customs, how law and order is maintained and so on.
The next section deals with Guilds, with a colourful page showing all the guild badges, and notes on how the guilds operate. If you have a good grasp of mediaeval European history, all of this is familiar territory - and if you don't it makes a good primer, clearly explained. Guilds control virtually all trades (it was a difference of opinion with a guild that got my character kicked out of Thay!) and they wield significant power within urban locales. This section is followed by one on Economics, which includes the coins to be found on Hârn and extensive price lists for just about everything you might want to purchase... and then looks at typical incomes: can you afford what you want to buy? And then there are taxes. And tolls. And guild dues if applicable. There's no escaping those.
The discussion moves on to Trade and then to Religion. Here it gets interesting. The GM needs to decide if the gods are real or not. As far as folk living on Hârn it makes no different, they think they are real anyway. Only the GM knows if anyone's listening to their prayers or if they are deluded. Whatever you decide, there's plenty of detail on the various deities worshipped in Hârn. This section provides organisations and cults to join, as well as doctrines to discuss, and the (real or perceived) differences between the gods which are played out on the mortal realm.
The next section is History. The first inhabitants of Hârn were the Earthmasters, but very little is known about them. Since their day, lost to legend, several waves of people have arrived, with humans but the last, arriving some 2,000 years ago. Since then history consists of wars and power struggles, that have shaped the landscape as it is known today. Empires rose and fell, kingdoms were established... the usual sweep of history, but all told in a vivid manner, you can imagine younglings hearing all this from their tutors.
Now, while Hârn as a setting is independent of ruleset (although you might want to try HârnMaster that was written for it) you might want to tweak characters to suit the setting, so there's a selection of tables to determine where and when characters were born (and even into what race...). This rules-ish section continues with information on things like travel times around Hârn and how the weather works.
If you want an incredibly detailed and realistic setting for your adventures, start here! You'll soon be hooked, and wanting to delve more deeply into the experience that is Hârn.